A remarkable feature of the nervous system is its ability to adjust stereotyped behavioral responses in a context dependent manner. For example, sudden and intense acoustic stimuli evoke the evolutionarily conserved startle response. While the execution of the acoustic startle response is extremely stereotyped, response initiation is modulated in a context-dependent manner. For example, repeated presentation of a startling stimulus suppresses the startle response, representing a simple form of learning known as habituation. Similarly, response threshold and thus startle sensitivity are also modulated in a context- dependent manner, such that for fear-conditioned animals otherwise sub-threshold stimuli are now sufficient to evoke a startle response. In humans, startle modulation -as measured by habituation or startle sensitivity- is impaired in several neuropsychiatric disorders, including schizophrenia, attention deficit disorder, Huntington's disease, obsessive-compulsive disorder (OCD) as well as in patients suffering from post-traumatic stress disorders (PTSD). Despite its importance, the molecular mechanisms underlying acoustic startle modulation are poorly understood. Zebrafish show a remarkable behavioral plasticity, and we have previously shown that larvae exhibit modulation of the acoustic startle response- including sensitivity and habituation- with landmark behavioral and pharmacological characteristics. Moreover, the neural circuit elements executing the zebrafish startle response have been defined, and due to its transparency larvae are amenable to genetic and opto-genetic manipulations. Using an automated system, we conducted the first forward genetic screen in vertebrates to identify genes required for acoustic startle modulation. We identified 24 mutants with defects in startle responsiveness to sub-threshold acoustic stimuli, defects in short-term habituation, or defects in both. None of these 24 mutants exhibit any gross morphological defects or defects in startle performance. By applying whole genome sequencing to two of these mutants, we identified gene mutations in the adaptor-related protein complex 2 sigma 1 subunit (ap2s1), and in the vertebrate specific pregnancy associated plasma protein-a (papp-a), respectively. Importantly, neither of these genes has previously been implicated in regulating habituation or startle sensitivity, suggesting that many of the remaining (un-cloned) 22 mutants may encode genes with critical roles in startle modulation previously not implicated in this process. Here, we propose to apply whole genome sequencing to the remaining 22 mutants to genetically map and identify the underlying causative mutations. Combined our results will increase the scientific impact of these mutants for the entire neuroscience community by creating an unparalleled platform for integrative studies to decipher the molecular and neural mechanisms underlying 'normal'startle modulation, and provide insights into neuropsychiatric disorders associated with impaired startle modulation, such as schizophrenia, OCD and PTSD.

Public Health Relevance

Modulation of behavioral responses is a fundamental process, yet the underlying molecular mechanisms are poorly understood. This proposal aims to determine the molecular identity of 22 zebrafish mutants with defects in startle modulation. This is directly relevant to the study of mental health and drug abuse, because deficits in startle modulation are a central feature of several cognitive disorders, including as schizophrenia, OCD and PTSD.